Standard television signal receiving systems typically include automatic gain control (AGC) apparatus for maintaining the magnitude of a signal applied to a detector stage substantially constant over a wide range of received signal levels. Such AGC apparatus typically generates an AGC signal as a function of the amplitude of the horizontal sync component of a detected baseband video signal component. The AGC signal is used to increase or decrease the gain of radio frequency (RF) and intermediate frequency (IF) amplifiers as an inverse function of the magnitude of the received television signal. Television signal AGC systems of this type are well known as described, for example, in U.S. Pat. No. 4,761,687--Rumreich.
Much attention has been devoted recently to the development of high definition television systems that encode and decode television information, e.g., image, audio and synchronizing information, in digital form. A transmitted digital television signal differs from a standard NTSC analog television signal in that it appears to be a random sequence of pulses, resembling a random noise signal of varying amplitude, and it may lack an RF carrier. Thus it may be difficult or impossible to use standard AGC techniques effectively for a digital television signal, due to the lack of an easily identifiable AGC reference component such as an RF carrier or a horizontal sync component similar to that of a standard NTSC television signal.
In recognition of these factors, a copending U.S. patent application Ser. No. 832,126 of T. M. Wagner et al., filed Feb. 6, 1992, describes a root-mean-square (RMS) AGC detector network suitable for use in a television receiver responsive to a signal representative of digital information. This AGC network may be advantageously used in a high definition quadrature amplitude modulated (QAM) digital television system having a spectrally shaped amplitude versus frequency response of the type shown in copending U.S. patent application Ser. No. 650,329 of Hugh Edward White. In that system, a transmitted high definition television signal lacking an RF carrier and an easily identifiable NTSC-type horizontal sync component is divided into high priority information that is intended to be received with high reliability, and lower priority standard information. The high priority information and the low priority information are conveyed as separate QAM carrier signals within different portions of the television signal frequency spectrum. The high priority information exhibits a narrower bandwidth and significantly larger amplitude than the low priority information.